1. Field of the Invention
The instant disclosure relates to a semiconductor element and manufacturing method thereof; in particular, to a memory device with novel structural bit lines and method for manufacturing the same.
2. Description of Related Art
Semiconductor memory device is an electronic data storage device that uses the electrical property of capacitance to store the bits of data. In recent years, miniaturization of memory cells has been achieved in semiconductor devices such as DRAM, and thus an increase in the number of memory cells connected to a bit line has caused a performance problem such as an increase in bit line capacitance.
DRAM is one type of volatile memory including plural memory cells. Each of the memory cells has a transistor and a capacitor coupled to the transistor. The electronic connection between neighboring memory cells can be achieved via word lines and bit lines.
In general, traditional memory device comprises a substrate, plural word lines embedded in the substrate, an insulating (oxide) layer disposed on the word lines, and plural bit lines disposed on the substrate. It is notable that each traditional bit line consists of a first polysilicon layer, a second polysilicon layer, and a conducting layer in a stacking arrangement. However, in the production of high-density integrated circuits, the miniaturized traditional bit lines may cause bigger contact resistance due to its narrow dimension, such that bit line loading is generated by lower current of the memory cells. Further, the subsequent formed word lines has coupling and pipe issues due to its step height.
Although the problem of high contact resistance can be reduced by increasing the junction depth of the word lines, but the increment of the junction depth can cause further problems of short channel effect and junction leakage. Furthermore, although the word lines with heavy dopant doped shallow junctions can prevent the problems of short channel effect and junction leakage, but are not arranged to achieve the bit line loading effect due to the restrict of solid solubility.